Special commercial uses of normal paraffins require that the normal paraffins contain an especially low concentration of aromatics. By normal paraffins, it is meant straight-chain, linear or unbranched paraffins. One of these special uses is the manufacture of detergents made from alkylbenzenes, in which C.sub.10 -C.sub.22 normal paraffins are dehydrogenated to olefins that are then used to alkylate benzene. The problems with aromatics in the normal paraffins, particularly aromatics having the same carbon number as the normal paraffins, arise during the alkylation step because of the occurrence of two side-reactions: first, the ring of the aromatic can react with an olefin to produce a heavy, dialkyl benzene by-product, and second the side-chain of the aromatic can be dehydrogenated and react with benzene to produce a heavy, biphenyl by-product. Either by-product is not suitable for detergents. These side-reactions result in waste of valuable feedstocks, costs for separation and disposal of by-products, and economic loss. For these reasons, there is sometimes a preference that the concentration of aromatics in normal paraffins used for commercial production of detergents be less than 0.005 wt-% (50 wppm) of the normal paraffins.
The most plentiful, commercial source of C.sub.10 -C.sub.22 normal paraffins is crude oil, in particular the kerosene-range fraction. By "kerosene-range" is meant the boiling point range of 360.degree.-530.degree. F. (182.degree.-277.degree. C.). This fraction is a complex mixture comprising normal paraffins, iso-paraffins, and aromatics from which the normal paraffins cannot be separated using conventional distillation. Depending on the type of crude from which the hydrocarbon fraction is derived and the carbon number range of the fraction, the concentration of normal paraffins is usually 15-60 wt-% of the feed and the concentration of aromatics is usually 10-30 wt-% of the feed. There may be more unusual feed streams which have aromatic concentrations of only 2-4 wt-% of the feed.
The separation of various hydrocarbonaceous compounds through the use of selective sorbents is widespread in the petroleum, chemical and petrochemical industries. Sorption is often utilized when it is more difficult or expensive to separate the same compounds by other means such as fractionation. Examples of the types of separations which are often performed using selective sorbents include the separation of para-xylene from a mixture of xylenes, unsaturated fatty acids from saturated fatty acids, fructose from glucose, acyclic olefins from acyclic paraffins, and normal paraffins from isoparaffins. Typically, the selectively sorbed materials have the same number of carbon atoms per molecule as the non-selectively adsorbed materials and very similar boiling points. Another common application is the recovery of a particular class of hydrocarbons from a broad boiling point range mixture of two or more classes of hydrocarbons. An example is the separation of C.sub.10 to C.sub.14 normal paraffins from a mixture which also contains C.sub.10 to C.sub.14 iso-paraffins.
One of the principal prior art processes for the selective removal of the aromatics from the kerosene-range fraction employs a sorption process that separates the normal paraffins and the iso-paraffins. The sorbent used in this process has pores which the normal paraffins can enter, but which the aromatics, like the iso-paraffins, cannot enter because their cross-sectional diameter is too great. Contacting a kerosene-range feed with the sorbent produces a raffinate stream containing almost all of the iso-paraffins and aromatics that were in the feed, and a sorbent loaded with sorbed normal paraffins. Then, contacting the loaded sorbent with a desorbent stream produces an extract product containing almost all of the normal paraffins in the feed. But, sorbents used in this process are not ideally selective for normal paraffins, and where the sorbent comprises a crystalline zeolite and an amorphous binder, the binder itself may be selective for aromatics. Consequently, a small portion of the feed aromatics is rather tenaciously sorbed on the surfaces of the sorbent and ultimately appears as a contaminant in the extract (normal paraffin) product. With a typical kerosene-range feed and a commercial sorbent, the concentration of aromatics is usually 0.15-0.50 wt-% (1500-5000 wppm) of the extract product, which is sometimes unacceptably high for production of commercial detergents.
A variation on the process described in the preceding paragraph can reduce the concentration of aromatics to about 0.05 wt-% (500 wppm) of the extract product. The distinguishing feature of this process variation is the contacting of the sorbent with a flush stream after contacting the sorbent with the feed stream and prior to contacting the sorbent with the desorbent stream. The flush stream contains a compound, typically another aromatic hydrocarbon, which desorbs some of the feed aromatics that had become sorbed on the sorbent, but does not desorb normal paraffins. The effluent from the flushing step is combined with the raffinate stream, meaning that the desorbed aromatics ultimately appear in the iso-paraffin product instead of the extract (normal paraffin) product. Unfortunately, some of the feed aromatics are not desorbed by the aromatic flush compound, and moreover during some abnormal circumstances some of the aromatic flush compound can even appear as an aromatic contaminant in the extract product. Therefore, even when a flush step is used with a typical kerosene-range feed and a commercial sorbent, the concentration of aromatics is usually about 0.05-0.08 wt.-% (500-800 wppm) of the extract product, which is still ten times higher than the current preference of some producers of commercial detergents.
Another prior art process can reduce the concentration of aromatics in the kerosene-range fraction to the required concentration, but it has serious economic drawbacks because it performs the removal of aromatics from the normal paraffins independently of the removal of iso-paraffins from the kerosene-range fraction. Initially, this process removes the isoparaffins from the kerosene-range fraction, thereby producing a stream containing normal paraffins and aromatics. Then, the removal of the aromatics, which employs a sorbent that preferentially sorbs aromatics, begins with the sorbent loaded with a desorbent. The sorbent is contacted with the normal paraffins and aromatics, thereby desorbing the desorbent, producing a raffinate stream containing normal paraffins and the desorbent, and leaving the sorbent loaded with aromatics. Next, the sorbent is contacted with the desorbent, thereby producing an extract stream containing the aromatics and the desorbent. But, in order to recover the normal paraffins as product, to discard the aromatics, and to recycle the desorbent, two distillation columns are needed----one for fractionating the raffinate stream and another for fractionating the extract stream. These two distillation columns along their associated reboilers, condensers, and other equipment, significantly increase the capital and operating costs of this process for the removal of the feed aromatics, making it economically unattractive.